What is the Beaufort scale?

Posted on September 17, 2012 by Weather Guys Editor

The Beaufort scale is a method of estimating wind speed based on the general condition of the surface of a large body of water with respect to wind waves and swell. This scale allows sailors to estimate the wind speed just by observing the state of the sea surface.

The scale has a long history, but was finalized in 1805 by Rear Admiral Sir Francis Beaufort, an Irish hydrographer in the British Royal Navy. It was officially first used during the voyage of Charles Darwin on HMS Beagle. The Beaufort scale can also be applied to conditions on land, but it is most often associated with the sea state.

The modern day Beaufort scale consists of 13 numbers ranging from 0 to 12. A Beaufort force 0 is assigned to calm winds where the water surface is smooth. A Beaufort force 12 occurs with waves greater in height than 46 feet and the sea is completely white with foam and spray with greatly reduced visibility. Such conditions are associated with wind speeds of greater than 74 mph, which are hurricane-force winds.

The National Weather Service defines sustained wind speeds of 39-54 mph as a gale, and forecasters typically issue gale warnings when winds of this strength are forecast. A Beaufort force 6 is a near gale with wind speeds between 25-30 mph and includes white foam from breaking waves that begins to be blown in streaks along the wind direction. A Beaufort force in the range of 6 to 7 is designated as strong winds; 8 to 9 as gale force winds; and 10 to 11 as storm force winds.

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What’s so funny about climate change?

Posted on September 12, 2012 by Weather Guys Editor

In his acceptance speech at the Republican National Convention, Republican presidential candidate Mitt Romney jokingly upbraided his opponent for campaigning on climate change and the impact it’s having on the planet.

“President Obama promised to begin to slow the rise of the oceans and heal the planet,” Romney said to a mix of laughter and boos from the assembled crowd. “My promise is to help you and your family.”

This column avoids politics, but we are equipped to speak about science.

Sea level rise is a real problem. The oceans are rising as the planet warms up, in part because the volume of water is expanding due to the extra heat, and in part due to the ongoing melting of polar ice.

The composition of our atmosphere has changed as a result of human activity on the planet — changed to the point where the climate itself has responded with its own changes.

These changes may well affect our ability to produce adequate food for a growing global population while putting our national commercial infrastructure at greater risk to extremes in the day-to-day weather, to name just two possible ramifications.

Climate change is a serious issue, and policymakers would do well to plan for the challenges it poses to our way of life.

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What is the source of energy for hurricanes?

Posted on September 3, 2012 by Weather Guys Editor

Hurricanes are large weather engines, and any engine needs energy to run.

The secret energy source of a hurricane is the large latent heat of water. Air over the tropical oceans is drier than you might think. Although both the air and water may be warm and calm, evaporation can take place because the air is not at 100 percent relative humidity.

Silently and invisibly, water changes from liquid to vapor and enters the atmosphere. The energy required to make this change comes from the sun, and this energy is lying in wait — latent — ready to be released when the vapor is condensed into liquid again. This happens in rising air in a cloud or thunderstorm.

However, this process alone is not enough to power a hurricane. A hurricane adds fuel to its own fire by drawing surface air toward its low-pressure center. The tight pressure gradient nearer the center means that the winds grow stronger as the air approaches the eye. The faster the wind blows, the more evaporation takes place (this is why you blow-dry wet hair or hands instead of merely warming them).

Increased evaporation means more water vapor in the air and more energy ready to be liberated in the hurricane’s thunderstorms as water vapor condenses. In short, evaporation and condensation of water are the keys to understanding the power of tropical cyclones.

How strong is the engine that powers a tropical cyclone? The energy released by condensation in a single day in an average hurricane is at least 200 times the entire world’s electrical energy production capacity. Part of this energy is expended reducing the central pressure of the storm and strengthening the winds.

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What are sundogs?

Posted on August 27, 2012 by Weather Guys Editor

On a day with high ice clouds, you are likely to see shiny, colored regions at either side of the sun. These are sundogs, an optical effect caused by refraction and dispersion of the Sun’s light through ice crystals. When the light rays strike the boundary between the air and water, like an ice crystal, several things can happen. Some rays are turned back in the direction from which they came, the familiar process of reflection. Other rays are transmitted into the crystal. Some of the transmitted rays change direction, a process known as refraction.

Sundogs appear because ice crystals in the shape of hexagonal dinner plates tend to drift downward with their flat bases parallel to the ground. The sunlight passes through the crystal and refracts sideways. If the Sun is low enough in the sky, you see spots of bright light on one or both sides of the sun, depending on where the clouds are. Refraction causes blue light to be bent more than red light, and so sundogs can show a spectrum of colors with red nearest the Sun.

Sundogs are usually 22 degrees away from the sun, or about a hand width from the center of the Sun when your arm is fully extended. Sundogs are often accompanied by a halo around the sun. A halo is a white ring that encircles but does not touch the sun. It is an optical phenomenon that also owes its existence to refraction of light by ice crystals. Because the light must shine through a fairly uniform layer of ice crystals that are thin enough to let light through, halos are usually associated with high, thin cirrostratus clouds.

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Are there fall weather changes beyond turning leaves and falling temperatures?

Posted on August 20, 2012 by Weather Guys Editor

As we head into the second half of August a subtle transition in our weather begins to occur — a transition that is probably hard to detect at first but that eventually becomes very obvious and then lasts for approximately eight months.

We are not talking about the gradual reduction in daytime high temperatures or the increasingly cooler to cold nights, though these are also beginning to invade. Instead, we are talking about the nature of the storms that deliver our precipitation.

Throughout the summer (even in this drought year), most of our precipitation comes in the form of thunderstorms wherein large amounts of precipitation fall in short amounts of time from what we call convective clouds. Most often these storms have lifecycles of only a few hours and drop precipitation over a relatively small area.

As we transition to late summer/early autumn, the thunderstorm frequency abruptly decreases and precipitation tends to occur in persistent, light to moderate rain events that will sometimes last an entire day. This mode of precipitation is associated with the passage of what are known as mid-latitude cyclones — storms that live for over a week during which time they can cover an area the size of 10 states.

As they progress across the country, these mid-latitude cyclones can drop precipitation (rain or snow) over enormous portions of the country. Though not entirely missing from summertime precipitation, such events are definitely the exception rather than the rule in the summer.

Category: Seasons
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